KABUL (Reuters) – U.S. and Afghan military officials hope that months of heavy fighting in southern Afghanistan have enabled them to head off a bloody offensive from the Taliban this spring as U.S. forces prepare to begin their withdrawal.

But serious doubts remain about whether U.S. and NATO troops, seeking to shift control to a growing but green Afghan army in 2011, can permanently weaken a tenacious insurgency that is strengthened by sanctuaries in neighboring Pakistan and has branched out into new areas of Afghanistan.

The first half of 2011 will be a make-or-break period for the Obama administration, which plans to start drawing down its force of about 100,000 in July, and for other NATO nations that are also looking to end combat activities in Afghanistan.

In the years since the Taliban government was toppled in 2001, insurgent attacks have usually slowed during the cold winter months, when snow piles up on mountain passes and militants slip across the Pakistani border to rest and reequip.

But there has been intensified fighting this winter, in part because of mild weather and also because of a surge of U.S. troops into southern Afghanistan, the Taliban heartland. A senior U.S. defense official in Washington, speaking on condition of anonymity, said the arrest or killing of some insurgents in 2010 may have weakened the Talibanâ€™s ability to bounce back this spring when fighting is expected to accelerate.

â€œThe real test will be how well they fight, how effective they are … Are they able to push us out?â€ the official said.

General David Petraeus, head of U.S. and NATO forces in Afghanistan, and other commanders have spent weeks touting their progress in driving insurgents from cities and other strategic areas of the south.

U.S. officials say that, more important than the frequency of continuing violence in the south, are signs Afghans are resuming normal life, sending children to school, traveling, trading.

Yet the war has become more bloody than ever. More than 2,300 foreign soldiers and many thousands more Afghan civilians have been killed since 2001.

Security has also deteriorated in parts of northern and eastern Afghanistan, where the al Qaeda-affiliated Haqqani network and other groups have intensified their attacks.

And despite years of trying, foreign forces have been unable to control the porous border with Pakistan, where the Talibanâ€™s senior leaders are believed to live.

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Zaher Azimi, spokesman for the Afghan Defense Ministry, said he expected the Taliban to try this spring to interrupt plans to gradually put Afghans in the lead, a process which President Hamid Karzai hopes will be concluded by the end of 2014.

â€œOur prediction is that the new year will be a bloody one but Afghan and other forces will have the upper hand,â€ Azimi said.

Because the Taliban have been weakened, he said, they would mostly be limited to planting roadside bombs — long the most effective weapon of insurgents — or launching suicide attacks.

To ensure the Taliban cannot fight their way back into areas cleared in 2010, the U.S. official said troops would use newly arrived tanks and perhaps new artillery to go after militants in less populated areas of Helmand.

â€œThere is a certain value to fracturing (the Talibanâ€™s) mid-level leadership … but what it also does is diminish any sort of central control,â€ said Joshua Foust, a U.S.-based analyst who blogs on central Asian security.

â€œIn the long run, that also diminishes the chances of a negotiated end to the fighting, something both President (Barack) Obama and General Petraeus say must happen.â€

Karzai has committed to seeking reconciliation with the Taliban but there are few signs such talks have any traction.